Under special operating conditions voltage surges of limited duration can occur in the electrical system of a vehicle. Causes of these voltage surges can include a so-called load dump, that is, an interruption of the connection between the generator and vehicle battery during the ignition process. The generator reacts to the sudden drop in load with a voltage increase, whose magnitude and trend is essentially dependent on the speed of the generator and the type of interruption. A pulse-like voltage surge is characteristic of a load dump. Causes include incomplete fastening or oxidation of the battery terminal and can be caused by a line break.
Battery discharge is critical for the electronic components connected to an electrical system of a vehicle, since formation of voltage transients with peak values up to 100 V can occur. However, in addition to protection of electronic components, the load dump can also be critical with respect to safety. A power window in operation or a sliding roof drive under each operating state must be capable of being switched off, so that the hazard of injury by jamming is ruled out as much as possible. Known devices for remote-operated power windows and sliding roof drives are therefore geared toward immediately switching off the motor after recognition of a load dump. Disengagement generally occurs through a relay. Immediately after the voltage surge, however, the motor accelerates and accepts increased current. If the switching process occurs in the initial phase of the voltage surge, during the disengagement process, because of the voltage increase and the increased motor current, an arc can occur over the opening contacts of the relay. In a simple design of the relay, it can even happen that the relay contacts melt and the switching device fails. The jamming protection required by safety engineering for the remote-operated actuator is then no longer guaranteed.
In order to protect assemblies in a vehicle electrical system from overvoltages, special components, like varistors, are known. However, their use requires not only additional space, but is often ruled out in automotive technology for cost reasons.
In order to protect electronic components in an electrical system of a vehicle from transient overvoltages without additional protective devices, a specially designed control unit is proposed in US 2002/0149261 A1. The control unit continuously monitors the electrical system for overvoltages. If the system voltage surpasses a stipulated limit value, loads, like glow plugs, seat heating or the heating device of the rear window, are connected to the electrical system. The overvoltage surge caused by a load dump is counteracted by this centrally controlled artificial loading of the electrical system. This protective measure, however, presumes that additional loads are present and can still be engaged on occurrence of the overvoltage.